Radio receiver performance is highly dependent on the selectivity of the receiver front end. Front-end selectivity is generally provided by a bandpass filter. Bandpass filters come in a variety of structural and topological arrangements. Most bandpass filters include a plurality of reactively coupled resonators which are coupled at a center frequency. Generally, the resonators are coupled to each other by reactive components, such as capacitors, inductors, or a combination thereof.
Front-end filters may have fixed bandwidths or they may have tunable bandwidths. Filters having fixed bandwidths are typically used in narrow band receivers where the operational frequency is limited to a narrow range. On the other hand, filters having tunable bandwidth may be used in wideband receivers where the range of operational frequency is substantially large and covers a wider bandwidth.
FIGS. 1 and 2 show prior art tunable preselector circuits 100, 200 described in U.S. Pat. No. 5,392,011 assigned to Motorola, Inc. and which is hereby incorporated by reference. Filter 100 provides a high-side injection tunable filter while filter 200 provides a low-side injection tunable filter. These filters are typically used to provide either low-side or high-side injection protection in single band radio operation.
Dual band radio operation presents a different set of logistical problems in front end radio receiver design. Though prior art high-side or low-side filter circuits may provide a certain degree of selectivity on the opposite side, this has been insufficient to meet dual band requirements. The standard filtering solution for dual band radio designs is to use two separate filters, one providing high-side injection protection and the other providing low-side injection protection. FIG. 3 shows a block diagram of the front end portion of a typical dual band receiver circuit 300. In operation, a radio frequency (RF) signal is received at antenna 302 and switched through one of two separate paths 304, 306. Path 304 provides high-side injection protection and includes a switch 308 and a filter 310. Path 306 provides low-side injection protection and includes a switch 312 and filter 314. Amplifier 316 amplifies the filtered signal and mixer 318 mixes the amplified signal with a local oscillator frequency. Filters 310 and 314 can be implemented with circuitry such as that shown in FIGS. 1 and 2 as well as other circuits known in the art to achieve the low-side and high-side injection protection. However, it would be highly advantageous to reduce the number of paths down to a single path through the use of a single preselector. A single filter tunable capable of both high-side and low-side injection protection would further provide the benefits of reduced complexity and reduced parts count.
Accordingly, there is a need for a single filter circuit capable of providing sufficient low-side injection and high-side injection protection. Such a circuit would have applications in a variety of communication systems including receivers and transmitters.